The reason we have to disbelieve her is that she spends two-thirds of SK misleading Harry, to point him at the Mothers so he will retrieve the Unravelling. And that so far the results of her actions have left her with the Paranet trusting her and working with her.
She was in a very difficult place there and had a clear motive for what she was doing. Her working for Summer was no secret either. She did what was expected and helped Harry where it counted.
Harry had to get the unraveling anyway otherwise the mothers had not given it to him and the later misdirection was double layered, Harry was just dimm witted as usual.
After that she was not under the same pressure and in white night they had both grown a little. Harry in Summer Knight is still very immature with unrealistic expectations from people following with resentment. His story did not consider the difficulty of Elaines position which is ironic considering Harry’s later relation with Mab.
She simply had no motives for misdirection anymore.
Why on earth not ?
She was close to Harry, they had a relation and they had soulgazed. Justin could never know exactly how close so he could not predict how she would really react when he started his magic on Harry. That would make her a risk and risks are to be avoided if possible. It was possible to avoid that risk.
Either by arranging it that she was not there, as he did when he enthralled Elaine, or by enthralling Elaine first.
Harry could point to the day it happened with Elaine which was extra evidence.